PS3 will have them as well if the user base grows. Right now it's a matter of security through obscurity.

There may already be hacks. It's not Live that is hacked but the game code itself. IW and Activision made a conscious decision to make the MP for the MW2 game peer-to-peer. Users can introduce hacks and mods into the game because of this. With dedicated servers a client check could be initiated at any time during a client/server session, identify hacks, and kick/ban modders.

The PS3 has a big enough user base.

I'm thinking its because the Xbox's OS is a lot easier to work with than the PS3 OS.